The single-phase p-q theory for the purpose of active power filtering in the case of single-phase loads is dealt with here. A simple modification is proposed to develop a generalised single-phase p-q theory that can be utilised under the condition of distorted utility voltage. A systematic study is presented by realising both direct and indirect current control techniques. The simulation as well as the digital signal processor (DSP) (DS1104 of dSPACE) based experimental results are discussed. The developed single-phase shunt active power filter (APF) prototype is tested under different operating conditions with different loads to evaluate the full capabilities of the proposed generalised theory for practical uses. The shunt APF reduces the source current total harmonics distortion (THD) from 27.2 to 3.4% under a distorted supply voltage with a THD of 16.2%.